1. Field of the Invention
The present invention relates to a process of producing a vinyl chloride polymer, and more particularly to a process of producing a vinyl chloride polymer having an average polymerization degree as low as 850 or less which is suitable for producing a vinyl chloride polymer in high quality while preventing deposition of polymer scale.
2. Description of the Prior Art
A vinyl chloride polymer is generally produced by suspension polymerization of vinyl chloride monomer or a mixture of vinyl type monomers mainly made up of vinyl chloride monomer in an aqueous medium in the presence of an oil-soluble polymerization initiator and a dispersion stabilizer, and if necessary, a chain transfer agent. Depending on the type of the polymerization initiator used at that time, the polymerization initiator affects greatly the pattern of heat generation, the polymerization time, the productivity, and the deposition of polymer scale during polymerization as well as the quality and characteristics of the polymer, such as initial coloration and heat stability.
In recent years, it is attempted to shorten the polymerization time in order to improve the productivity of vinyl chloride polymers. As one technique of shortening the polymerization time, it is thought to increase the amount of the polymerization initiator to be added. However, as the amount of the polymerization initiator is increased, there occurs a deterioration in the quality of the polymer, especially a deterioration in initial coloration, an increase in deposition of polymer scale in the polymerization vessel.
Particularly in the case of producing a vinyl chloride polymer having a polymerization degree as low as 850 or less, it is required to effect polymerization usually at a temperature of 58.degree. C. or more. However, when the polymerization initiators, which have been conventionally used, such as di-3,5,5-trimethylhexanoyl peroxide, lauroyl peroxide, t-hexyl peroxypivalate and t-amyl peroxypivalate, are used, there occurs disadvantages that the quality of the polymer is lowered and polymer scale is liable to be deposited in the polymerization vessel.
In order to prevent deposition of polymer scale in the polymerization vessel, it has hitherto been carried out that the inner surfaces of the polymerization vessel are coated with, as a polymer scale deposition preventive agent, dyes, pigments, water-soluble polymer compounds, salts of nitrous acid, nitrogen-containing aromatic compounds, heterocyclic compounds, iodine compounds, pyrogallol derivatives, phenolic compounds and aromatic amine compounds. However, these conventional polymer scale deposition preventive agents can not suppress effectively polymer scale deposition in the polymerization vessel, especially gas phase part and gas phase-liquid phase interface part and thereabout during producing a vinyl chloride polymer having a low polymerization degree.